It is well known that many metallocenes can be useful for the polymerization of some olefins. Particularly it has been noted that metallocenes can be combined with aluminoxanes to produce catalyst systems of high activity for the production of polyolefins. A particularly interesting type of metallocene for olefin polymerization is the so-called bridged sandwich-bonded metallocene in which the ligand of the metallocene comprises two cyclopentadienyl-like groups connected through a bridging group. Some of the bridged sandwich-bonded metallocenes when used in the polymerization of olefins having three or more carbon atoms have been found to be useful for producing polymers having different types of microstructure as reflected by tacticity determined by spectroscopic techniques such as infrared and NMR. A discussion of such techniques is disclosed in Zambelli et al, J. Polym. Sci. Part C, 84, 1488, (1962); Lotz et al, Macro Molecules, 21, 2375, (1988); Haftka et al, J. Macromol. Sci. Phys., 830, 319, (1991); and Youngman et al, Macromol. Res., 2, 33 (1967).
It is well known in the art that the microstructure tacticity of a polyolefin molecule can have a significant effect upon the physical properties of the polymer. Other things which affect the polymer properties include the type of monomer, and comonomer if employed, the weight average molecular weight (M.sub.w) of the, polymer, the molecular weight distribution, and the composition distribution of the resin. Accordingly, for producing commercially desirable polymers, it is important to find metallocene catalysts which give the desired combination of polymer properties plus commercially practical polymerization activities.
Many sandwich-bonded bridged metallocenes have been at least proposed in the open literature and the patent literature and there have been some studies of the effects of varying the structure of the ligand used in the metallocene. One example of such a study is disclosed in the New Journal of Chemistry, Vol. 14, No. 6-7, pages 499-503 (1990). While the patent literature contains broad assertions regarding the particular types of polymers that will be produced with specific types of metallocenes, subsequent work has revealed that those generalizations are too broad.
For example, while U.S. Pat. Nos. 4,794,096 and 4,769,510 teach that bridged chiral, stereorigid metallocene catalysts are capable of producing polymers having high levels of isotactic microstructure, the only actual examples of such metallocenes are racemic ethylene bridged bisindenyl or bis-tetrahydroindenyl metallocenes which must be isolated from mixtures of the racemic and meso isomers by difficult, tedious, expensive fractional crystallizations or similar techniques. Further U.S. Pat. No. 4,892,851 shows that that the bridged, chiral, sterorigid metallocene cyclopentadienyl isopropylidene fluorenyl zirconium dichloride produces highly syndiotatic polypropylene rather than isotactic polyproplyene. Also Dr. Abbas Razavi in a paper at the SPO 92 meeting reported that the bridged, chiral, stereorigid metallocene racemic [bis(3-methyl indenyl)ethylene] zirconium dichloride yeilds a highly amorphous polypropylene rather than an isotactic polypropylene. The present inventors have also noted that certain bis-unsubstituted fluorenyl bridged metallocenes produce amorphous polypropylene having only low levels of isotaticity as determined by NMR.
An object of the present invention is to provide compositions comprising bis-fluorenyl bridged sandwich-bonded metallocene which are suitable for use in preparing isotatic polypropylene even without separation of racemic and meso isomers.
Another object of the present invention is to provide processes for polymerizing olefins using the special metallocene compositions.
In accordance with yet another aspect of the present invention, there is provided processes for producing novel isotactic polymers.
Other aspects, objects, and advantages of the present invention will become apparent from a review of the following disclosure.